1. Field of the Invention
The present invention relates to a toner used for the development of electrostatic images in the image forming process, for example, by electrophotography, as well as a manufacturing method thereof.
2. Description of the Related Art
Along with remarkable development of recent OA (Office Automation) equipment, image forming apparatus such as printers, facsimile units and copiers have been popularized generally. As the image forming apparatus, electrophotographic image forming apparatus of forming images by electrophotography has been often used. In the electrophotographic image forming apparatus, images are formed by utilizing a photoconductive material. Specifically, after forming static charges by various means on the surface of an electrophotographic photoreceptor having a photosensitive layer containing photoconductive material (hereinafter also simply referred to as “photoreceptor”), static charges are developed by supplying a toner to the surface of the toner receptor and the formed toner images are fixed to a transfer material such as paper thereby forming images.
The toner used for the development of static charges (hereinafter referred to as “toner for static charge development”) comprises a colorant dispersed in a resin having a binding property referred to as a binder resin and, optionally, contains various additives such as a charge controller. The toner is charged by triboelectric charging and supplied while being carried on a developing roller or the like to the surface of the photoreceptor.
The manufacturing method of the toner for electrostatic image development is generally classified into a dry process and a wet process. The dry process includes, for example, a pulverization method of kneading a binder resin, a colorant, etc. and pulverizing and granulating the obtained kneaded resin product. While the dry process has been industrially used generally, since the toner obtained by the dry process has a relatively wide grain size distribution, it tends to vary the charging performance.
In a case of forming images by using the toner with varied charge performance, it results in a problem of lacking in the applied charged amount to result in a toner not transferred to the transfer material, lowering the transferability of toner images to the transfer material and resulting in lowering of the image density or white background fog. Furthermore, in a case of a color toner, a problem of causing color shedding to images is arises. The white background fogging is a phenomenon that the toner is deposited to a portion of the transfer material which should be a white background with no deposition of the toner.
For suppressing the variety of the toner charging performance in the dry process, for example, in the pulverization method, it is necessary to apply classification after granulating by pulverization thereby making the grain size distribution narrow, which results in another problem of increasing the manufacturing cost.
On the other hand, since the wet process has an advantage capable of manufacturing a toner with a narrow grain size distribution and having less variety of the charging performance compared with the dry process relatively easily, the wet process has often been adopted recently for the manufacture of the toner. For the wet process, there have been proposed methods, for example,
(i) a suspension polymerization method of polymerizing a monomer of a binder resin dispersed by a suspension stabilizer in a dispersion medium such as water under the presence of a colorant and incorporating the colorant in the resultant binder resin particles to obtain a toner;
(ii) an agglomeration method by a emulsion polymerization of mixing a liquid resin dispersion and a liquid colorant dispersion formed by dispersing a colorant in a dispersion medium-to form agglomerated particles, and heating to fuse the agglomerated particles to obtain a toner;
(iii) a phase transfer emulsification method of dissolving or dispersing a water dispersible resin and a colorant in an organic solvent, adding thereto a neutralizing agent for neutralizing dissociation groups of the water dispersible resin and water under stirring, forming resin droplets incorporating the colorant or the like, and emulsifying them under phase transfer to form a toner;
(iv) a dissolving suspension method of dissolving or dispersing a toner material containing a binder resin and a colorant in an organic solvent to which the binder resin is soluble, mixing the resultant solution or the liquid dispersion with an aqueous solution of an inorganic dispersant, for example, of a less water-soluble alkaline earth metal salt such as calcium phosphate or calcium carbonate thereby conducting granulating, and then removing the organic solvent to obtain a toner refer, (for example, refer to Japanese Unexamined Patent Publications JP-A Nos. 7-152202 (1995), 7-168395 (1995), 7-168396 (1995), 7-219267 (1995), 8-179555 (1996), 8-179556 (1996), and 9-230624 (1997)); and
(v) an emulsifying dispersing method of dissolving or dispersing at least a binder solution and a colorant in a non-aqueous organic solvent to which the binder resin is soluble, emulsifying and dispersing the obtained solution or liquid dispersion in an aqueous liquid dispersion, and then removing the organic solvent to obtain a toner (for example, refer to Japanese Unexamined Patent Publications JP-A 7-325429 (1995), 7-325430 (1995), 7-333890 (1995), 7-333899 (1995), 7-333901 (1995), and 7-333902 (1995)).
However, the wet processes also involves problems to be solved. For example, the suspension polymerization method (i) involves a problem that the monomer of the binder resin, polymerization initiator, suspension stabilizer, etc. remain in the inside or on the surface of the obtained toner particles to bring about variety of the charging performance of the toner particles. In order to suppress the variety of the charging performance, while it is necessary to remove residues, it is extremely difficult to remove the monomer, polymerization initiator, suspension stabilizer, etc. intruded in the inside of the toner particles. Furthermore, since the removal of the residues requires complicated steps, they result in the problem of increasing the toner manufacturing cost. Furthermore, since the monomer of the binder resin, etc. gives a large burden on the environments, it requires a processing facility for appropriately treating them, which further increases the production cost. Furthermore, in the suspension polymerization method, since the polymerizing reaction is accompanied during granulating, it also has a problem that the binder resin usable therein is restricted to acrylic resins.
Furthermore, in the agglomeration method by emulsion polymerization (ii), since the toner is manufactured by agglomerating the binder resin and the colorant and heat fusing them, this results in a problem that toner particles of a uniform composition can not be formed stably.
Furthermore, in the phase transfer emulsification method (iii), the dissolving suspension method (iv), and the emulsification dispersion method (v), since an organic solvent is used for dissolving or dispersing the binder resin, they result in a problem that a small amount of the organic solvent remains in the obtained toner particles to change the dispersion state and the composition for each of the ingredients in the toner particles on every production lots to vary the charging performance of the toner particles. Furthermore, since the shape of the toner particles is changed by the level of pressure, that is, degree of depressurization upon removing the organic solvent, temperature, time, etc., toner particles of a uniform shape can not be formed stably which may possibly vary the charging performance.
Furthermore, in a case of using the organic solvent, since the amount for each of the ingredients contained in the toner particles, that is, the composition of the toner particle changes depending on the solubility or the dispersibility of the binder resin to the solvent, it is difficult to manufacture a toner having a desired characteristic at a good reproducibility. Furthermore, since the organic solvent gives a significant burden on the environments, the methods (iii) to (v) require a facility of appropriately disposing the removed organic solvent, which increases the production cost of the toner.
Furthermore, in the dissolving suspension method (iv) and the emulsifying dispersing method (v), since the binder resin is granulated by dissolving in the organic solvent to which the binder resin is soluble and mixing with a dispersant or an emulsifier, a resin soluble to the organic solvent, for example, a linear resin of a relatively low molecular weight, for example, with a weight average molecular weight of about 10,000 to 50,000 is used as the binder resin. Accordingly, when images are formed by using the toner produced by the solvent suspension method or emulsifying and dispersing method, this results in a problem of causing hot offsetting phenomenon. The hot offsetting phenomenon means such a phenomenon that the toner is melted excessively during fixing in a hot roller fixing method of conducting fixing by heating the toner by the fixing heat roller, and a portion of the molten toner is carried away being fused on the fixing heat roller and transferred to a subsequent transfer material.
For the method of preventing the hot offsetting phenomenon, while an anti-offsetting solution such as a silicone oil has been coated to the fixing heat roller, the method involves a problem of complicating the apparatus and making the maintenance troublesome.
As a method of preventing the hot offsetting phenomenon with a view point of the toner material, it may be considered to improve the anti-hot offsetting property of the toner by using a resin of high molecular weight with a weight average molecular weight, for example, of about 50,000 to 500,000 or a resin containing a gel ingredient insoluble to tetrahydrofuran (hereinafter referred to as tetrahydrofuran insoluble component or tetrahydrofuran insoluble ingredient) for the binder resin. However, since the resin is not dissolved or less dissolved to the organic solvent, it is difficult to granulate toner particles when intending to manufacture the toner by the solvent suspension method or emulsifying and dispersing method using such toner. Even when the toner particles could be granulated, it is difficult to form toner particles of a desired composition at a good reproducibility. Particularly, the composition of the resin used as the starting material can not often be maintained and since only the ingredients soluble to the solvent are contained in the obtained toner particles, it is difficult to suppress the hot offsetting phenomenon.
As a method of manufacturing the toner incorporated with a toner resin containing the tetrahydrofuran insoluble component, it has been proposed a method of obtaining a toner by mixing a mixture formed by kneading a binder resin, a colorant, a wax, and an organic solvent in a wet process with an aqueous medium to emulsify and form a resin particles incorporating a colorant or the like, and separating the resin particles from the liquid medium followed by drying (refer to Japanese Unexamined Patent Publication JP-A 2002-6550). However, in the method disclosed in JP-A 2002-6550, since the organic solvent is used, it results in a problem that the organic solvent remains in the toner particles to vary the charging performance like in the methods described in (iii) to (v) described above.
As a method of manufacturing a toner without using the organic solvent, it has been proposed a method of manufacturing the toner by mixing and mechanically. dispersing a molten product obtained by heat melting a kneading product of a synthetic resin (binder resin) having ionic groups and a colored pigment and an aqueous medium containing a material for neutralizing the ionic groups and heated to a temperature higher than the softening point of the synthesis resin, then rapidly cooling the same to prepare an aqueous dispersion of fine colored resin particles and drying and separating the fine colored resin particles from the aqueous dispersion solution (for example, refer to Japanese Patent No. 3351505).
However, the technique disclosed in Japanese Patent No. 3351505, involves a problem that formed fine colored resin particles (hereinafter also referred to as toner particles) adhere to each other to grow in the dispersing step and the cooling step. For preventing the growing, it is-necessary to strictly control conditions such as a liquid temperature of the liquid mixture of the molten product and the aqueous medium. For example, in Example 1 of Japanese Patent No. 3351505, the temperature of the liquid mixture has to be cooled rapidly from 165° C. to 65° C. within 10 sec. Actually, it is extremely difficult to apply such control which makes the manufacturing steps complicated.
Furthermore, in the technique disclosed in Japanese Patent No. 3351505, since the binder resin is emulsified by neutralizing the ionic groups in the binder resin with the neutralizing material to disperse the same in the aqueous medium, it has a problem that the resin usable therein is restricted only to those resins having ionic groups. Furthermore, a reverse neutralizing step of resuming the ionic groups of the binder resin in the formed toner particles into the original shape is necessary after the granulating, and this increases the manufacturing steps. Furthermore, since it is difficult to apply reverse neutralization to the ionic groups in the binder resin incorporated in the toner particles, this also results in a problem that the ionic groups remain in the toner particles to vary the charging performance.